Recent development in wireless communication has been focused on multiple-input and multiple-output (MIMO) communication systems. In a MIMO system there are one or more antennas in an array at a transmitter and one or more antennas in an array at a receiver. MIMO systems take advantage of the spatial diversity created by the separation between each antenna in an array in order to improve performance in terms of increased reliability, range, and capacity of data transmission. MIMO systems may enable improved performance by using multiple antennas at a receiver to detect the cumulative effect of a signal and any reflected copies of the signal that traverse multiple paths from a transmitter to the receiver. MIMO systems may also enable improved performance by using multiple antennas at a transmitter to send copies of the same signal from each antenna such that they constructively interfere in a desired direction, referred to as beam forming, to cumulatively provide a stronger signal than any one antenna could transmit to the receiver.
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique that may be used in wireless communication systems to improve spectral efficiency and provide for resistance against multi-path interference. OFDM divides a frequency channel into multiple overlapping orthogonal sub-carriers with data symbols modulated onto each sub-carrier. Since each of the sub-carriers is orthogonal, the peak of each sub-carrier, or a tone, coincides with the nulls of all of the other sub-carriers such that a sub-carrier can be sampled at its strongest point without any interference from the other sub-carriers. By taking advantage of this orthogonality, the entire frequency range of each channel may be used without causing interference to adjacent frequency channels such that guard frequencies, that are used in legacy modulation techniques, do not have to be used with OFDM. Also, since each frequency channel is divided into multiple sub-carriers, each sub-carrier may operate at a low symbol rate. The low symbol rate creates a strong resistance against multi-path and multi-symbol interference.
Taking advantage of both of these recent advances in technology, MIMO communication systems that transmit data using OFDM are now being designed. A standardization process for how these types of wireless systems should communicate is in progress with 802.11n. The goal of the 802.11n standard is to provide faster wireless communication of at least 100 Mbps net throughput that can reliably communicate over greater ranges than the 802.11a/g predecessors, but still supports communication with systems using the legacy standards. A receiver in an 802.11n compliant system may receive several different types of packets corresponding to the legacy standards and the 802.11n standard. Each of the different types of packets may have different formats that may need to be processed differently by the receiver. A need exists for an 802.11n compliant receiver to be able to identify the type of packet that it is receiving in order to enable proper processing of the data contained in the packet.